In the first half of the 20th century, genetics and developmental biology were considered separate disciplines. The word "epigenetics" was coined by Conrad Waddington in 1942. He formed it from the Greek word "epigenesis", which initially stood for the effect of genetic processes on development. Today the epigenetic regulation of genes can be defined as changes in gene activity, which are inherited through mitotic and/or meiotic cell cycles, without changes in DNA sequence. It is currently considered that at least three biological systems start, maintain, or alter epigenetic code; including DNA methylation, histone modification, and gene silencing with implementation of non-coding RNA. Since epigenetic alterations are not tied to changes in DNA sequence, the inheritance thereof is connected to a probability of reversibility to a certain degree. The aforementioned degree, however, relies on the very same mechanism that caused the epigenetic changes. An example of plant’s epigenetic gene regulation is vernalization – the ability of a plant to develop generative organs when influenced by low temperatures. Before vernalization, flowering is suppressed with the expression of the FLC gene (Flowering Locus C). After the plant has been exposed to low temperatures for some time, the expression of the FLC gene is silenced through the modification of chromatine structure in this gene locus.